1. Field of The Invention
This invention relates to an image intensifier converting an X- or .gamma.- ray image into a visible image and more particularly to an improvement of an input screen of the image intensifier.
2. Brief Description of The Prior Art
An image intensifier converting high energy radiations such as .gamma.-ray or X-rays into bright visible rays comprises and input screen converting the image of the high energy radiations into a photoelectron image and an output screen converting the photoelectron image into a visible image.
It is desirable that the visible image has high resolution which depends mainly upon converting fidelity of the input screen.
The input screen has a substrate usually made of aluminum which transmits effectively such radiations. On the substrate an alkali halide phosphor layer luminescing effectively by the radiation is formed by vacuum deposition. Further on the phosphor layer a photocathode for example cesium antimonide (Sb-Cs) sensitive to the luminescence of the phosphor is deposited.
Hitherto, to improve the resolution of the input screen, a cracked phosphor screen having a plurality of phosphor blocks (a bundle of columnar crystals) separated by cracks from each other is known. In this screen light generated in a phosphor block is scattered only within own block and cannot travel to other blocks. Namely each block (not columnar crystal) has light guiding effect. This phorphor screen is prepared by depositing a phosphor material of cesium iodide on an aluminum substrate and thereafter heating them to generate cracks in the deposited phosphor by means of a difference of thermal expansion coefficients between the substrate and the phosphor.
However this input screen has following drawbacks:
(1) As cracks are generated by a strain caused by a difference between the temperature of the substrate and that of the surface of the phosphor layer which is higher than the former, they are liable to be generated from the upper surface of the phosphor layer. Consequently it is difficult that cracks reach near the substrate. Because phosphor blocks formed by cracks generated from substrate's side are few, light guiding effect of the phosphor layer is insufficient. Resolution of the X-ray image intensifier having such input screen is therefore 28-30 lp/cm.
(2) As cracks generated by heat treatment of the screen, it is difficult to obtain the input screen having stable quality.
A method comprising a step of impressing a metallic gauze (e.g. copper gauze) upon an aluminum substrate and a step of depositing cesium iodide on the gauze thereby making a phosphor layer composed of phosphor blocks is well known as another method for making a cracked phosphor layer. However this method has some disadvantages.
(1) Because the surface of the phosphor layer becomes uneven owing to depressions of the phosphor surface corresponding to the meshes of the gauze, this uneveness affects badly the characteristics of a photocathode deposited on the surface.
(2) It is difficult to impress the metallic gauze upon the substrate without generating any folds because the substrate is normally domed.
A phosphor layer composed of a plurality of phosphor blocks (a bundle of columnar crystals) which are deposited on a gauze-like uneven surface of a substrate prepared by conventional photoetching technique is another prior art for obtaining high resolution. However, because separation between adjacent blocks is not distinct, light guiding effect is insufficient. Moreover uneveness of upper surface of the phosphor layer has a bad influence upon characteristics of a photocathode deposited on the layer.